1. Field of the Invention
The invention relates to high thermal conductivity substrates and more particularly to sintered diamond substrates having improved electrical properties.
2. The Prior Art
The need for high thermal conductivity substrates has been well known for a long time. For example, microwave transmission systems employ devices such as the solid state IMPATT diode oscillator. These diodes are reverse biased and often require high current densities to sustain high frequency oscillations. Flux densities of energy dissipated is greater than 10.sup.5 Watts/cm.sup.2 which is three orders of magnitude larger than that for conventional rectifier diodes. In nearly all microwave devices, heat generated during operation at high power decreases efficiency and dissipation of the generated heat is a critical factor in limiting operation. Other devices having critical heat dissipation requirements include advanced power amplifiers. Hence, a substrate material with a thermal conductivity significantly greater than that of copper is desired. At room temperature, oxygen-free, high thermal conductivity (OFHC) copper has a thermal conductivity, k, of approximately 4 Watts/cm.degree.C. Sintered beryllium oxide, another commonly used substrate, has a thermal conductivity of only about 1.8 Watts/cm.degree.C. but has excellent dielectric properties, i.e., the electric field created in the substrate produces only a minimum heat component.
Type IIa single crystal natural diamond has the highest k value of any known material at room temperature and has been used for microwave device substrates. Typical values of k for the type IIa are between 20 and 30 at room temperature. Natural single crystal diamond, however, has not been used extensively because of significant cost and extreme difficulty in shaping.
The use of sintered polycrystalline diamond composites for the same purpose has met with consistent difficulty. The primary problem appears to be that the desired thermal conductivity is often accompanied by undesirable electrical characteristics. While the diamond-carbon is electrically resistant, all presently known polycrystalline diamond composites are infused with a sufficiently large quantity of graphite-carbon or other material which interferes with predictable electrical conductivity and dielectric properties.